Electroluminescent apparatus



B. KAZAN ETAL ELECTROLUMINESCENT APPARATUS Filed OOL. 5l, 1957 March 14, 1961 beffen/f.

INVENToRs. 50g/111112 TTRNEX f' l VAS/BLE SCREEN f 2,975,294 n ELECTRLUMINESCENT APPARATUS Benjamin Kazan, Princeton, and James E. Berkeyheiser, ir., Trenton, NJ., assignors to Radio Corporation of America, a corporation of Delaware Filed Oct. 31, 1957, Ser. No. 693,663 Claims. (Cl. Z50-213) This invention relates to electroluminescent apparatus, and more particularly to an improved electroluminescent device for the intensication of images.

Recently, electroluminescent devices have been developed for amplifying light, X-rays, and other forms of radiant energy. These devices are usually panel structures comprised of superimposed layers of photoconductive material and electroluminescent material. An electric field is established across the superimposed layers. Under excitation by radiant energy, as for example in the form of an X-ray image, the impedance of elemental areas of the photoconductive layer is decreased so that a potential distribution corresponding to the image projected upon the photoconductive layer is established on the layer of electroluminescent material. ln the presence of excitation of the photoconductive layer, the electroluminescent layer becomes illuminated and an amplified image Vcorresponding to the image projected on the photoconductive layer is produced.

The nature of the material providing the photoconductive layer is such that, in the absence of radiation thereon for a period of time, the conductivity of the material drops to a very low level. This conductivity is so low that the layer of electroluminescent material cannot provide output light. A predetermined amount of radiation is therefore required on the photoconductive layer to increase the conductivity thereof to an operating level. This predetermined amount of radiation may be referred to as the threshold radiation required for the light amplifier device to begin to amplify signals in the form of radiation which is incident thereon. For example, as much as ten seconds may be required for a dim X-ray image, such as might be obtained in X-ray fiuoroscopic examinations, before the threshold radiation sufficient to permit amplification in the electroluminescent device is provided.

Electroluminescent devices are particularly important for X-ray iiuoroscopic examinations. Such devices in the form of light amplifier panel structures may be used instead of the conventional fiuorescent screen in an X-ray iluoroscope. The primary deterrent to maximum utilization of the X-ray iiuoroscope technique is that, with conventional fluorescent screens, it is impossible to obtain an image of sufficient intensity for adequate visual observation through tlie use of X-ray exposures consistent with safety to the patient. To obtain visual images of sufficient intensity with conventional fluorescent screens, X-ray exposures are required which would be injurious to the patient under examination. While the use of light amplifier devices in X-ray iiuoroscopy reduces the total exposure to X-rays necessary to provide a bright image for visuai observation, the patient may be needlessly exposed to the threshold radiation which docs not provide signals capable of being amplified by the light amplifier devices. As mentioned above, a period of time, which may be several seconds, is required before a visible image is provided by a conventional light amplifier device. Consequently, the time for fluoroscopic examinations may be prolonged. This is particularly the case if several different images inust be viewed successively, since traces of the first image must be completely removed before the subsequent image can be viewed. This involves allowing the photoconductor to fall in conductivity below the threshold level. The present invention provides an elec- 2,975,294 Patented Mar. 1.4, 1961 troluminescent image intensification or light amplification apparatus wherein any problems resulting from the threshold radiation effect are eliminated.

Means have been previously proposed for initially providing the threshold radiation to photoconductive devices by means of auxiliary sources of radiation. For example, an auxiliary priming lamp may be placed so that radiation therefrom is incident upon the surface of the photocoriductive layer in a light amplifier panel. This auxiliary light is turned on before each exposure of the light amplifier panel to an image to be amplified. Such methods for priming the light amplifier for providing the threshold radiation have not been entirely satisfactory. With auxiliary sources of illumination of the type heretofore provided it is very difficult to uniformly illuminate the surface of the photoconductive layer. Without uniform illumination different quantities of radiation are applied to different parts of the photoconductive layer. Thus, the output images produced by the amplifier devices may be extremely non-uniform. Another drawback of previous devices for providing threshold illumination is that they interfere with the projection of an image on the photosensitive surface of the light amplifier panel. For example, an auxiliary lamp positioned to provide a more uniform illumination of the photosensitive surface of the light amplifier may be in the path of the projected image, thus absorbing the light of the projected image and decreasing the intensity of the projected image to a greater extent to one part thereof than another part thereof. The light from the projected image may also be scattered due to the presence of the projected image. This non-uniform reduction in the intensity of the projected image produces large variations in the brightness of the amplified image.

In accordance with the present invention, an improved electroluminescent device for the intensification of images projected thereon is provided Which eliminates the problems due to threshold radiation effects without the difiiculties and compiexities of prior devices. Briefly described, the invention provides a novel combination of an improved auxiliary or priming source of illumination and an electroluminescent device. For example, this auxiliary source of illumination may be a panel structure comprising a layer of electroluminescent material which is coextensive with the light amplifier panel and is disposed adjacent the photosensitive surface thereof. The image to be amplified with the light amplifier panel passes through the auxiliary electroluminescent device without scattering or absorption so -that an undistorted visible image may be produced by the light amplifier. Since the electroluminescent device is coextensive with and adjacent to the photosensitive surface of the light amplifier device, uniform illumination of the photosensitive surface results.

it is therefore an object of the present invention to provide improved electroluminescent image intensification apparatus.

It is another object of the present invention to provide an improved electroluminescent light amplifier apparatus wherein novel means are provided to eliminate the problems due to the threshold radiation effect.

It is a further object of the present invention to` provide improved X-ray fluoroscopic apparatus requiring less exposure to X-ray illumination than heretofore needed.

lt is a still further object of the present invention to provide electroluminescent light amplifier apparatus having improved auxiliary illumination means for providing threshold radiation without non-uniformity of illumination or interference with the image to be amplified.

The novel features of the invention as well as additional objects and advantages thereof, will be understood 3 more fully from the following description when read in connection with the acco-mpanying drawings, in which:

Fig. l is a front view of an improved electroluminescent image intensifier device provided in accordance with the present invention; i

Fig. 2 is a cross sectional view on line 2 2 on Fig. l of the improved electroluminescent light amplification device shown in Fig. l;

Fig. 3 is a fragmentary front view of the device shown in Fig. 4;

Fig. 4 is a fragmentary cross sectional View on line 4 4 on Fig. 3 of an improved electroluminescent image intensifier device provided in accordance with another embodiment of the present invention; and

Fig. 5 is a schematic drawing of a circuit for operating devices of the type shown in the foregoing figures.

Referring now to the drawing, and particularly to Figs. l and 2 thereof, there is shown an improved electroluminescent image intensifier or light amplifier device provided by the present invention. While the device is referred to as a light amplifier device it will be observed that the device shown in Figs. l and 2 is adapted for the amplification of X-ray images. It will be appreciated that the present invention is useful in providing electroluminescent image intensification devices suitable for use with various forms of radiation. The term, light amplifier is used in a general sense to identify image intensiiication apparatus of the type useful in the present invention.

The device shown in Fig. 2 includes a container 10 of plastic or other non-conductive material. It is desirable that the container be of opaque material so that it will be light-tight. One end of this container has a large rectangular opening therein. The other end is closed. The container 10 also has two pairs of side walls. Each end wall has a surface area very much larger than the surface area of any one of the side walls. The container 1G is essentially a panel. The length and Width of the end of the container may each be several feet, whereas the container may be less than an inch thick. X-rays from a source, such as used in a conventional X-ray fluoroscopic examination apparatus may be used to illuminate the enclosed end of the container 10. Since the container is made of thin, plastic material which is substantially transparent to X-rays, these X-rays will pass therethrough with relatively little attenuation. Ordinarily, an X-ray image will be projected onto the closed end of the container. This image will be amplified in the device. An intensified image may be viewed on the surface of the device disposed adjacent the open end of the container 10.

Adjacent the open end of the container 10 there is disposed a light amplifier device 24 of the type which has been heretofore developed. The construction of this de vice 24 is described in greater detail in an application liled in the name of Benjamin Kazan on December 30, 1954, Serial No. 478,707. This light amplifier device 24 is a panel structure of narrow cross section. A transparent support member which may be a glass plate 13 is disposed directly adjacent the open end of the container 10. A transparent conductive film 14, which may be a hlm of tin chloride or iinely sprayed silver paint, is applied to a surface of the glass plate 13. A layer of electroluminescent material 15, such as may comprise many of the common phosphore, is applied over the conductive `film 14. Phosphors of various types, such as copper activated zinc sulphide, Zinc beryllium silicate and the like, depending on the desired color output, are suitabie. in preparing the electroluminescent layer, the particles of phosphor material are mixed with or are embedded in a light transmitting-or insulating material, 'for example a plastic, lacquer, wax or the like. The thickness of the layer 1S of electroluminescent material may be from one-to three-thousands of an inch. A light opaque insulating layer 16 may be applied over the therefore substantially light tight.

' in the light amplifier panel structure 24.

electroluminescent layer 15. The region in the container between the closed end thereof and the opaque layer is Over the opaque layer 16 there may be applied, if desired, a layer 17 of currentdiusing material. This current diffusing material may comprise cadmium sulphide which has been made conductive by first adding cadmium chloride and then heating the mixture to about 700 degrees centigrade for twenty minutes. The layer 17 of current-diffusing material is followed by a layer 18 of photoconductive material sensitive to the X-rayradiation. Cadmium sulphide, cadmium selenide and lead sulphide may be used as the photoconductive material, for example. The photoconductive material may be applied in dry powder form. However, in the illustrated light amplilier panel 24, the photoconductive material is mixed with a suitable plastic binder such as ethyl cellulose or methyl methacrylate. Grooves having a triangular cross-sectional shape may be formed in the layer 18 of photoconductive material for permitting the X-ray radiation or incident light to penetrate ytherein and excite the photoconductive material along its entire depth. It is desirable that the maximum width of the grooves should not exceed the width of an element of the picture to be observed. It is desirable that the grooves should Ibe small in width in order to preserve image resolution. The apex of each groove of the layer of photoconductive material 18 is coated with conductive elements or lines 19. These lines may be formed by painting with the silver or any other conductive pain-t. The transparent conductive film 14 is connected Vto a terminal 20. Each line conductor is connected, through a common connection, to another terminal 21. These terminals 21 and 22 are adapted to be connected to a source of voltage for energizing the light ampliiier panel structure.

An auxiliary source 25 of illumination for providing the threshold radiation for the light amplifier device is disposed immediately adjacent the surface of the photoconductive layer 18 of the light amplier structure 24. This auxiliary source 25 includes a sheet 26 of non-conductive and transparent material, such as a plastic. A sheet of the plastic, polystyrene, will be found suitable. A transparent layer 27 of conductive material is applied on a surface of the transparent non-conductive sheet 26. This layer 27 of conductive material may be aluminum which is evaporated on the surface of the sheet 26. A layer 28 of electroluminescent material is applied over the layer of conductive material 27. This layer of electroluminescent material may be prepared in a manner similar to the Vlayer 15 of electroluminescent material It may be constituted of particles of phosphor material. A phosphor is selected which will produce radiation, such as visible light, of the typeto which the photoconductive material of the layer .18 is especially sensitive. For example, a copper activated compound of zinc, sulphide, and selenium, which is designated by the chemical formula, Zn(S:Se)Cu, may be suitable. This phosphor produces a yellow-green light to which the cadmium sulphide material of the photoconductive layer 18 is sensitive. The surface of the phosphor layer 28 may be sprayed with a thin layer of silver paint, which is X-ray transparent, to provide another layer 29 of conductive material. rl`=he layers 27 and 29 of conductive material provide electrodes for establishing an electric field across the layer 2S of electroluminescent material. Connections may be brought from a pair of terminals 3G and 31 to the conductive layers 29 and 27, respectively. These terminals 30 and 31 may be connected through appropriate circuitry to a source of voltage for energizing the electroluminescent device which will produce the auxiliary source of illumination 25.

-In operation the auxiliary source 25 of electroluminescent radiation is energized by voltage applied across the terminals 30 and 31. The source v25 produces illumiexample, 0.01 to 0.1 foot lamberts of illumination for -approximately one second may be suficient toy provide the requisite threshold radiation for the panel 24. After this initial radiation is applied by the auxiliary source 25, the auxiliary source may be disconnected from the source of voltage. X-ray signals which thereafter are incident upon the photosensitive surface of the light amplifier 24 will be amplified. The patient, whose X-ray image is being observed, will not be unnecessarily exposed to provide the threshold radiation. None of the radiation signals capable of contributing to the image formed by the light amplifier device will be needed to provide the threshold radiation. Thus, the light amplifier device 24 will be an efficient amplifier over a large range of levels of radiation which may occur in an X-ray image.

In operation, X-rays incident upon the photosensitive surface of the light amplifier panel 24 from an external source (represented in Fig. 2 by the dashed arrows designated X-RAY) located on the same side of the photosensitive surface 18 as is the auxiliary source 25 are projected through the electroluminescent auxiliary light source 25. The attenuation of the X-rays by the auxiliary source 25 is negligible. It will be observed that the support layer or sheet 26 is constituted of plastic material which is substantially transparent to X-rays. This sheet is the thickest element of the electroluminescent auxiliary source 25, and may, for example by approximately between one to ten-thousandths of an inch in thickness. The thickness of the transparent layer 27 is only a fractional part of one-thousandth of an inch. Similarly, the thickness of the other layer of conductive material 29 is less than one-thousandth of an inch. Consequently, these layers, have,A in total, a very insignificant effect in attenuating the X-rays. Since lthe electroluminescent layer 2S of phosphor material may be approximately one-thousandth of an inch in thickness, it is also substantially transparent to X-rays. Consequently the X-ray image which is projected on the device passes through the auxiliary source 25 Without appreciable attenuation.

The front end of the container 10, which is adjacent the layer 29, may be eliminated if desired.

The spacing between the surface of the photoconductive layer 18 in the light amplifier device 24 and the back surface of the plastic sheet 26 in the electroluminescent light source 25 may be very close. The distance between the two aforementioned surfaces may be only a few thousandths of an inch. Consequently, the illumination provided by the auxiliary source 25 is applied substantially without attenuation to the photosensitive surface of the light amplifier device 24.

It is a feature of the invention to uniformly illuminate the photosensitive surface of a light amplifier device. Since the electroluminescent layer in the auxiliary source 25 is substantially coextensive with the surface of the layer 18 of photoconductive material in the light amplifier 24, the photoconductive layer 18 will be uniformly illuminated all over its surface.

It will be observed that the light amplifier includes a layer 16 of opaque material. Consequently the illumination from the auxiliary source 25 cannot pass through the light amplifier 24 and be visible to the observer who is viewing the amplified image on the surface of the electroluminescent layer 15 so as to adversely affect the brightness of the image.

The apparatus shown in Figs. 3 and 4 is similar to the device shown in Figs. l and 2. However, the device of Figs. 3 and 4 is more suitable for amplifying visible images which are projected thereon. A container 40, constructed of material similar to the material of the container 10, encloses a light amplifier device 41 and an electroluminescent auxiliary light source 42.

The end of the container 4G which is adjacent the light amplifier 41 is open so that the amplified image on the surface of the eletcroluminescent screen in the light arnplifier may be viewed. This was the case with the container 10 shown in Fig. 2. Since a visible image is to be projected on the light amplifier 41 for amplification, the front end of the container 40 also has a large rectangular opening therein through which the visible image can be projected.

The construction of the light amplifier 41 may be identical to the construction of the light amplifier 24 shown in Fig. 2. Similarly, the layered construction used for the auxiliary source 25 may be used in the electroluminescent auxiliary source 4Z. The proximity of the adjacent surfaces of the auxiliary source 42 and the surface of the photoconductive layer `41 may be as close as possible. Therefore, the same relative disposition of the source 42 and the device 41 as was used in the case of the apparatus shown in Fig. 2 may be maintained in the device of Fig. 4. However, in order that the visible image may be transmitted through the electroluminescent auxiliary source 42, there are provided, through all of the layers of the auxiliary source `42, a plurality of closely spaced perforations 43.' These perforations 43 may be uniformly arranged, as indicated, or they may be arranged in a random manner. The largest number of perforations consistent with rigidity of the structure 42 may be used.

In operation, the light due to the Visible image will pass through the layers of the auxiliary source without attenuation by way of the perforations 43. It will be remembered, however, that a plastic sheet 44, is used las the base of the electroluminescent auxiliary source 42. This plastic sheet is coated with a very thin evaporated film 45. A layer of electroluminescent phosphor material 46 is then applied to the film 45. Over the phosphor layer 46, there is applied another layer 47 of conductive maferial, such as evaporated silver or aluminum paint. The plastic 'sheet is substantially transparent. Because of the thinness thereof, the conductive lms 4S and 47 also do not substantially attenuate the visible image as the image passes therethrough. The phosphor particles used in the electroluminescent layer `46 are dispersed in a plastic binder. This binder may be provided by a transparent plastic. Consequently, the plastic layer is translucent. The auxiliary source may operate properly when used with a visible image, even in the absence of the perforations 43. It is then desirable to place the auxiliary source directly adjacent or in contact with the light amplifier 41. However, through the use of the perforations 43, the attenuation of the image is largely reduced.

The operation of the device shown in Figs. 3 and 4 may be substantially identical to the operation of the device shown in Fig. 2. Other methods and means for operating the improved electroluminescent light amplifier device provided by the invention will bev discussed in connection with Fig. 5.

Fig. 5 illustrates in a schematic manner a circuit which may be used for operating the electroluminescent devices of the type shown in Figs. l and 2 or Figs. 3 and 4. A pair of terminals 5t) and 51 are provided which are adapted to be connected to the terminals 20 and 21, respectively. The terminals 20 and 21 are connected to the conductive layers which provide electrodes for the light amplifier panel. The terminals 50 and 51 are connected to a source of alternating current voltage. Alternating current voltage has been found more suitable for energizing a light amplifier panel. The frequency of this voltage may be 420 cycles per second.

Another pair of terminals 53 and 54 are provided which `are adapted to be connected to the terminals 30 and 3-1 of the auxiliary electroluminescent source. The terminal 51 is connected to an alternating current source '55. The frequency of this alternating current may be the same or different from the light amplifier. One side of the source 55 is connected to the terminal 54 '7 through a timer switch 56, shown illustratively as a switch element enclosed by the dashed box. A resistor 57 is connected across the switch 56.

In operation, the alternating current source continuously applies a voltage -across the light amplifier so that an electric field is established between the electrodes provided by the conductive layers and lines thereof. When an exposure is to be taken, the timer switch is closed. Consequently, full voltage of' the source 55 is applied across the auxiliary electroluminescent source. The phosphor layer or screen, therein, is therefore brightly illuminated. A large quantity of illumination is applied to the light amplifier device -at the beginning of each exposure. The light amplifier device is therefore primed by the immediate application thereto of sufcient illumination to provide the requisite threshold radiation. After a very brief period, as for example, approximately one second, the timer switch opens and the magnitude of the voltage between the terminals 53 Iand 54 is decreased due to tne voltage drop across the resistor 57. This decreased voltage causes a decrease in the level of iilumination of the electroluminescent screen in the auxiliary source. Thereafter, during the exposure, the illumination is maintained at a low level so as to sustain the threshold radiation and increase the sensitivity of the light amplifier due to the photoresponsive characteristics of the photoconductive material in the light amplifier are that the photoconductivity thereof will decay when the exciting radiation is removed or is reduced. `lf a dim image is projected on the surface of the photoconductive layer and the auxiliary illumination is removed, elemental portions of the photoconductive layer may decrease in photoconductivity below the threshold. This decrease in photoconductivity may give rise to reduced sensitivity of the amplifier for prolonged observation of dim images. Consequently, maintenance of the auxiliary illumination at a low level While the amplifier is exposed to the image will maintain the threshold radiation and prevent reduction in sensitivity of the photoconductive layer. Moreover, for subsequent exposure to different images, the intensity of the priming illumination from the auxiliary Source may be decreased, as may the duration of the intense priming illumination. The uniform illumination of the light amplifier by the apparatus of the present invention will prevent rapid decay of photoconductivity after the image' projected on the device is removed, thereby lengthening the persistence of the image on the screen of the amplifier and increasing the viewing time for limited X-ray exposures.

What is claimed is:

1. Electroluminescent apparatus comprising an electroluminescent light amplier device having a photore-sponsive surface on which images to be amplified are projected, and an electroluminescent panel having a luminescent surface on one side thereof disposed opposite to said photoresponsive surface, and a surface on the oppoSite side thereof exposed to said projected images, said panel having la plurality of perforationstherein extending through said surfaces for transmitting said projected images to said photoresponsive surface substantially without attenuation.

2. Electroluminescent apparatus comprising a container of opaque material having a pair of opposite end walls and two pairs of side walls, said end walls each having a surface area larger than the surface area of one of said side walls, one of said end walls having a large opening therein, an electroluminescent light amplifier device disposed in said container, said device including a layer of electroluminescent and a layer of photoconductive material, said layers being disposed between said side walls, a layer of opaque material disposed between adjacent surfaces of said layer of electroluminescent material and said layer of photoconductive material, the opposite surface of said layer of electroluminescent material being' disposed adjacent said end wall of said container having said opening therein to present a surface for viewing a projected image, the opposite surface of said layer of photoconductive material being located in said container facing said closed end wall, and an electroluminescent panel disposed in said container between said closed end wall and said opposite surface of said photoconductive layer, said panel extending between said side walls, said panel including a sheet of transparent, non-conductive material disposed opposite and adjacent to said opposite surface of said photoconductive layer, a layer of transparent conductive material over said sheet, a layer of electroluminescent material superimposed on said layer of conductive material, and a layer of conductive material over said layer of electroluminescent material.

3. Electroluminescent apparatus comprising a container having a pair of opposite end walls and two pairs of side walls, said end walls each having a surface area very much larger than the surface area of one of said side Walls, said end walls each having a large opening therein, an electroluminescent light amplifier device disposed in said container, said device including a layer of electroluminescent and a layer of photoconductive material, said layers being disposed between said side Walls, a layer of opaque material disposed between adjacent surfaces of said layer of electroluminescent material and said layer of photoconductive material, the opposite surface of said layer of electroluminescent material being disposed adjacent one of said end walls of said container to present a surface for viewing a projected image, the opposite surface of said layer of photoconductive material being located in said container facing the other of said end Walls, and an electroluminescent panel disposed in said container between said other end wall and said opposite surface of said photoconductive layer, said panel extending between said side walls, said panel including a sheet of transparent, non-conductive material disposed opposite and adjacent to said opposite surface of said photoconductive layer, a layer of transparent conductive material over said sheet, a layer of electroluminescent material superimposed on said layer of conductive material, a layer of conductive material over said layer of electroluminescent material, said panel having a plurality of perforations therein extending through the layers thereof, said perforations being disposed to transmit said projected image to said photoresponsive surface.

4. Electroluminescent apparatus comprising an electroluminescent light amplifier device having a photoresponsive surface, an electroluminescent auxiliary light source having a luminescent surface disposed adjacent and oppos1te to said photoresponsive surface, means for applying a source of voltage to said light amplifier device for continuously energizing said light amplifier device, and means for applying a voltage of a first magnitude to said auxiliary light source for a short period of time and for thereafter applying a voltage of a second magnitude less than said first magnitude for causing said auxiliary source to luminesce at successively lesser magnitudes of illumination.

5. Electroluminescent apparatus comprising an electroluminescent device having a photoresponsive surface responsive to radiations from a primary source of radiation external to said apparatus at one side thereof, and said apparatus including an auxiliary source of illumination as a part thereof also located on the same side of said photoresponsive surface as said external source, said auxiliary source having a luminescent surface disposed opposite to said photoresponsive surface, and said auxiliary source also being transparent to said radiations.

References Cited in the file of this patent UNITED STATES PATENTS 2,624,857 Magner Ian. 6, 1953 2,739,244 Sheldon Mar. 20, 1956 2,773,992 Ullery Dec. 11, 1956 2,802,963 Sheldon Aug. 13, 1957 2,883,556 lenny et al Apr. 21, 1959 

